I. Field of the Invention
This invention relates to an underlay composition for use in the fabrication of integrated circuits, and in particular to a lift-off process involved in such fabrication.
II. The Prior Art
One method used for the manufacture of integrated circuits involves the formation of vacuum deposited thin metal films which are etched in the presence of etch resistant photoresist layers to provide the selected pattern. This, in effect, involves the traditional photoengraving or photolithographic etching technique. However, with the continued miniaturization or semiconductor integrated circuits, to achieve greater component density and smaller units in large scale integrated circuitry, additional processes have been developed wherein photoresist layers are patterned over a substrate, metal is vacuum deposited over the photoresist layer and subsequently the photoresist is removed, leaving fine metal linework or patterns on the substrate.
As an example of such additional processes developed by the art to obtain fine minute resolution is a process wherein a bottom layer or underlay of a nonphotosensitive polymeric material is formed on a silicon substrate which may already contain previously patterned layers as well. Then a photoresist composition is deposited on the underlay and is exposed to a selected pattern of radiation such as electron beam or ultraviolet light. Openings are formed in the photoresist layer by development of the radiation exposed portions of the photoresist layer as well as aligned portions of the polymeric underlay. A thin metal film is then deposited on the undeveloped portions of the photoresist layer and on the substrate through the apertures formed in the photoresist and underlay.
Pattern formation of the deposited thin film layer on the substrate is achieved by subsequent removal of the photoresist/underlay composite together with the excess thin film by immersing the substrate in a solvent which dissolves the polymeric underlay. This just described method for the formation of patterned metallic films is referred to in the art as the "lift-off" process, and by such process, lateral widths of thin metallic films are deposited on silicon wafer substrates and spaced in the order of 0.5 mils or less. An example of a prior art teaching of such a lift-off process is contained in U.S. Pat. No. 4,004,044.
Polymeric compositions which are to be used as underlay material must be thermally stable at the temperatures, e.g., 210.degree.-230.degree. C., at which deposition of thin metallic films occurs to avoid decomposition and/or insolubilization of the polymeric underlay. Thermoplastic polyimide polymers may be used as underlay materials because of their high thermal stability, e.g. in excess of 400.degree. C. Polyimides are generally defined as polymers having the repeating imide linkage ##STR1## in the main chain and are derived from an aromatic dianhydride such as pyromellitic anhydride and an aliphatic or aromatic diamine.
Thermoplastic polyimides are well known to the art. An example of such material is commercially available and sold under the trademark XU 218 by Ciba-Geigy Corporation of Ardsley, New York. The polyimide is formed by first condensing benzophenonetetracarboxylic dianhydride with 5(6)-amino-1-(4'-aminophenyl)-1,3,3-trimethylindane (DAPI) and then heating at 25.degree. C. until it is fully polymerized. XU 218 is supplied as a solid powder, has a density of 1.2 grams per cubic centimeter and a glass transition temperature (Tg) of 320.degree. C.
Polyimide formulations used as underlay materials may have incorporated therein a radiation absorbing dye to improve linewidth resolution and uniformity in the deposited photoresist imaging layer. Thus, it has been determined that linewidth control problems can arise due to light scattering and reflection from the substrate-underlay interface during exposure of the photoresist imaging layer to radiation such as ultraviolet light. The light scattering and reflection can cause the photoresist to be exposed to an undesirable high dose of ultraviolet light, which in turn leads to undesirable linewidth variation. It has been found by the art, e.g. U.S. Pat. No. 4,362,809 that the light-scattering and reflection phenomenon can be reduced by incorporating in the polyimide composition a radiation absorbant dye. When the imaging source is ultraviolet radiation the dye used is one which will absorb light at a wavelength of 350-500 nanometers (nm), with a maximum absorbance preferably in the 400-500 nm range.
A polyimide solution commercially available from Ciba-Geigy which has been evaluated as an underlay material for use in lift-off processes is composed of a mixture of about 15 percent by weight XU 218 and 5 percent by weight Orasol Yellow 4GN a monoazo dye dissolved in a gamma-butyrolactone solvent. The dye dissolved in a gamma-butyrolactone solvent absorbs light in the 350-500 nm range.
One of the disadvantages to the use of the Orasol 4GN dye is that the dye exhibits marginal thermal stability at 230.degree. C. Deposition of the thin metal films is normally performed at this temperature whereby the dye is vulnerable to decomposition. The outgassing that can result from dye decomposition will cause undesirable dimensional changes or distortion of the deposited metal lines.
A second disadvantage to the use of the Orasol 4GN dye is that high concentrations of dye, e.g. in the order of 5 percent by weight, are required to prevent linewidth variations due to light scattering. The presence of the high dye concentration has been found to materially increase the time necessary to effect lift-off of the polyimide underlay film.
A further disadvantage in the use of the commercially available polyimide formulations is that the presence of high boiling point solvents such as gamma-butyrolactone (b.p. 203.degree. C.) causes "edge pull back" or volume concentration of the film after it is cast on the substrate. Edge pull back results in the formation of a 1 to 2 mm.diameter band of relatively thin (e.g. 1-1.2 microns) film material to form at the edges of the substrate as compared to the thickness of the remainder of the cast film, e.g., about 1.8-2.0 microns, normally applied as the underlay.
During processing of the coated substrates, as by argon sputter cleaning of film surfaces, the heat generated during such processing causes the relatively thin band of polymeric film at the edge of the substrate to be more firmly bonded thereto whereby removal of the underlay with conventional solvents is rendered impossible thereby preventing lift-off of the photoresist layer and the deposited metal layer.
A thermally stable underlay composition that would be thermally stable at temperatures in excess of 230.degree. C., which could be cast on semiconductor substrates without the occurrence of edge pull back and which could be easily and completely removed in lift-off processing would be highly advantageous and much prized by workers in the art.
It is thus an object of the present invention to provide a thermally stable underlay composition that can be used for fabrication of semiconductors at temperatures in excess of 230.degree. C.; which can be readily and completely removed in lift-off processing.